1/*
2 * Copyright 2013 Google Inc.
3 *
4 * Use of this source code is governed by a BSD-style license that can be
5 * found in the LICENSE file.
6 */
7
8#ifndef SkOnce_DEFINED
9#define SkOnce_DEFINED
10
11#include "include/private/SkThreadAnnotations.h"
12#include <atomic>
13#include <utility>
14
15// SkOnce provides call-once guarantees for Skia, much like std::once_flag/std::call_once().
16//
17// There should be no particularly error-prone gotcha use cases when using SkOnce.
18// It works correctly as a class member, a local, a global, a function-scoped static, whatever.
19
20class SkOnce {
21public:
22 constexpr SkOnce() = default;
23
24 template <typename Fn, typename... Args>
25 void operator()(Fn&& fn, Args&&... args) {
26 auto state = fState.load(std::memory_order_acquire);
27
28 if (state == Done) {
29 return;
30 }
31
32 // If it looks like no one has started calling fn(), try to claim that job.
33 if (state == NotStarted && fState.compare_exchange_strong(state, Claimed,
34 std::memory_order_relaxed,
35 std::memory_order_relaxed)) {
36 // Great! We'll run fn() then notify the other threads by releasing Done into fState.
37 fn(std::forward<Args>(args)...);
38 return fState.store(Done, std::memory_order_release);
39 }
40
41 // Some other thread is calling fn().
42 // We'll just spin here acquiring until it releases Done into fState.
43 SK_POTENTIALLY_BLOCKING_REGION_BEGIN;
44 while (fState.load(std::memory_order_acquire) != Done) { /*spin*/ }
45 SK_POTENTIALLY_BLOCKING_REGION_END;
46 }
47
48private:
49 enum State : uint8_t { NotStarted, Claimed, Done};
50 std::atomic<uint8_t> fState{NotStarted};
51};
52
53#endif // SkOnce_DEFINED
54